Implement AssumeRole API for Minio users (#7267)

For actual API reference read here

https://docs.aws.amazon.com/STS/latest/APIReference/API_AssumeRole.html

Documentation is added and updated as well at docs/sts/assume-role.md

Fixes #6381

cmd/signature-v4-utils_test.go

@@ -248,7 +248,7 @@ func TestGetContentSha256Cksum(t *testing.T) {
 		if testCase.h != "" {
 			r.Header.Set("x-amz-content-sha256", testCase.h)
 		}
-		got := getContentSha256Cksum(r)
+		got := getContentSha256Cksum(r, serviceS3)
 		if got != testCase.expected {
 			t.Errorf("Test %d: got:%s expected:%s", i+1, got, testCase.expected)
 		}